Coded track circuit signaling system



Dec. 15, 1953 J. Y. HOWARD CODED TRACK CIRCUIT SIGNALING SYSTEM 10 Sheets-Sheet 1 Filed July 15, 1949 INVENTOR.

Hi5 ATTORNEY.

Dec. 15, 1953 J. Y. HOWARD 2,662,971

CODED TRACK CIRCUIT SIGNALING SYSTEM Filed July 15, 1949 10 Sheets-Sheet 2 FIGJB.

by QVENTORI g Hi5 ATTORNEY.

FIG. 1C.

Dec. 15, 1953 .1. Y. HOWARD 2,662,971

CODED TRACK CIRCUIT SIGNALING SYSTEM Filed July 15, 1949 10 Sheets-Sheet 3 Mam Hi5 ATTORNEY.

Dec. 15, 1953 HQWARD 2,662,971

CODED TRACK CIRCUIT SIGNALING SYSTEM,

Filed July 15, 1949 10 Sheets-Sheet 4 UFIGI. 1D.

3-, gZVENTOR. :I MMVQMZW H15 ATTORNEY.

Dec. 15, J. Y. HOWARD CODED TRACK CIRCUIT SIGNALING SYSTEM Filed July 15, 1949 1O Sheets-Sheet 5 51 INVENTOR. t

0 l wwwt {5| a l BY 2 Mil |l [Km Hi5 ATTORNEY.

Dec. 15, 1953 J. Y. HOWARD CODED TRACK CIRCUIT SIGNALING SYSTEM 10 Sheets-Sheet 6 Filed July 15, 1949 Hi5 ATTORNEY.

Dec. 15, 1953 J. Y. HOWARD 2,662,971

CODED TRACK cmcux'r SIGNALING SYSTEM y Filed July 15, 1949 10 Sheehs-Sheet 7 HCI.1G.

IINVENTOR. izwm HIS ATTORNEY.

Patented Dec. 15, 1953 zicsz sn conc TR 'cK' CIRCUIT SIGNALING SYSTE James. Y. How r Gr Y x q to Gene e lwiirer S gn mr lmhes erg .agpligsnon l y-15, 1949, serm-noz-ioacoi 1-. The present-inventionrelates to automatic signalling systems for railroa'ds', and more particularly pertains to the application of coded: track circuits to the controlling ofwayside signals of a single track railroad. The signalling system described herein is of' the type commonly known as an absolute-permissive-block signalling systemand, as such, provides protectionagain'st opposing train movements" between head-block signals and also permits following train moves-with the customary block spacing protection.

The conventional coded track circuit provides for the transmission overthe-track rails ofpulses of current which are iiitermittently applied to one end of a track section with the spaced intervals between the pulsesknown as off periods.

Such a code is commonly termed a driven.

code, the rate at wa ts-inspects are appliedto the track circuit being" accordance with the currenttrafii'c conditions; At the other end of each track section is, in addition to the code responsive means which receives the driven code pulses, an inverse code transmitter which transmits code pulses upon-the track circuitdur ing the off periods of the driven code. latter code is known as an "inverse" code. At

the end of the tracksection sending the" drivencode, suitable meansarexiipl oyfed to receive-the inverse code pulses? k Generally speaking and without attempting to define the invention in exact detail; the various controls for the signals of the system described herein are transmitted both by driven and inverse codes with the-driven" codes being'of distinctive rates and the inverse" codes of distinctive polarities. Thus; each block fco'mprisingaportion of track between consecutive signals is to be clivided'byin d lating joints as its mi'd--- portion into two separate track sections; by means of the circuit organizationprovided} both driven and inverse codes are to be alternately applied to each of these-track sections. In this manner, the required number of signal controls may be transmitted in a symmetrical y tem-empl y s a m m mfl pm ni' W1 9 viding for approach lig ing of all the wayside signals. i v With the ac wav unoccupie the si na s have their mechanismsall operated to clear posi} tions, but the signal indication s'are not displayed if approach lighting is empIOyed. The driven codes transmittedinthe track circuits at this time are at a 180 rate andthe inverse codes are of a positive polarity.

Drivencodes are-always trammitted from each cut-section in both directions at code rates sea '75 rate code is transmitted as a carrier for the inverse code that is transmitted in the opposite.

direction, the caution-control of a; signalbeing only in response to a 120 code rate. Thus the control of the coderate to be transmitted to a signal from a cut sectionisselected in accordance with. the polarity of the inverse code received at the cut section astransmitted from the next signal in the rear for each direction of traffic. In this manner, any one of three code rates can be selectedfrom driven transmission in accordance with the polarity or absence of inverse code transmitted from the nextsignal in the rear. 1 I

'An' object of the present invention, is therefore to continuously transmit driven codes at selected code rates-in both directions from'the respective cut sectionsbetween signal locations for a direction when the signal subject to driven code control forthe' associated track section is at stop. It is proposed that upon the entrance of atrain into thestretch of single track between headblock locations a'tumble-downwill occur so as to place all opposing signals in that stretch of track at stop? A further object, therefore, is to provide in the case or. a tumble-downfor the t'ransmission',in the direction of movement of-the' trainer driven codes of a carrier rate to thereby permit the transmission of inverse codes in opposite direction. In addition, directional'stick relays are located at each sgnallocation so .that following train movements may be made with the usual blockspacin'g protection.

Other objects, purposes, and characteristic features of the presentinvention will be in part obvious from the accompanying drawings and in part pointed out"as' the"d'escription of themvention progresses.

In describing the invention in detail, reference will be made to the accompanying drawings. in

3 which like letter reference characters provided with distinctive numerals designate corresponding parts throughout the several views and in which:

Figs. 1A to 1G, inclusive, when placed end to end, illustrate a portion of single track railroad signalled in accordance wth the present invention;

Figs. 2A to 2J illustrate trafllc diagrams of the signalling system corresponding to different conditions of train movements; and

Fig. 3 illustrates the circuit arrangement at a single intermediate signal location.

For the purpose of simplifying the illustration and facilitating in the explanation, the various parts and circuits constituting the embodiment of the invention have been shown diagrammatically and certain conventional illustrations have been employed, the drawings having been made more with the purpose of making it easy to understand the principles and mode of operation than with the idea of illustrating the specific construction and arrangement of parts that would be employed in practice. Thus, the various relays and their contacts are illustrated in a conventional manner, and symbols are used to indicate connections to the terminals of batteries, or other sources of electric current, instead of showing all the wiring connections to these terminals.

The symbols and are employed to indicate the positive and negative terminals respectively of suitable batteries or other suitable sources of direct current and the circuits with which these symbols are used always have current flowing in the same direction.

Referring to Figs. 1A to 1G, inclusive, these figures, when placed end-to-end, illustrate a portion of single track railroad between two passing sidings indicated as A and B and including an additional portion of single track to the right of passing siding B. These figures also indicate that the even numbered signals 2, 4, 6, 8, l and I2 govern the passage of east bound trains; whereas, the odd numbered signals 8, 5, l, 9, II, and i3 govern the passage of westbound trains. The various head-block signals such as signals 2, 9, and H) are of the absolute type; whereas, all the other signals are of the permissive or Stopand-Proceed type. It will also be noted that the various track sections have been numbered in accordance with the number of the signal which governs the entrance of trains into that track section. For example, the track section governed by signal 4 has been designated by the reference character 4T.

Each of the track switches located at each end of a passing siding has associated therewith a switch circuit controller, the contacts of which are included in the circuit organization at that location. As an example, Fig. 1A indicates that a contact III of the switch IW circuit controller has been included in the circuit controlling the operation of the yellow-green repeater relay SYGP. These controllers operate in such a manner that they interrupt the circuit in which they are located only when the switch with which they are associated is outof its normal position. In this connection it should be noted that all of the switches shown are normally positioned so as to permit main line trafllc and each contact of a switch circuit controller is, therefore, positioned to close the circuit in which it is included.

In Figs. 2A to 2.], inclusive, the different signal conditions of the portion of railroad track illustrated have been indicated for various possible traffic conditions. In these diagrams it has been expedient to employ conventional symbols. Thus, trains have been indicated in block form and the conditions of the various signals have been represented by the position of a semaphore arm. The semaphore arms have been shown in heavy line for those signals which are illuminated, the position of the arm indicating the particular aspect of the signal which is then being shown. A semaphore arm which is shown as a light solid line in dicates the controlled condition of that signal but indicates that that particular signal is not illuminated.

It is contemplated in the present invention that the driven codes will be of certain distinctive rates as, for example, 75, 120, and pulses per minute, although other suitable rates might as well be employed. These driven codes are generated from suitable code generating devices such as code oscillators located at each cut-section.

These code oscillators have been denoted in Fig. 1B by the reference characters 5-1501, 5-l20CT, and 5-l80CT. It should be understood that each of these code oscillators has a selfcontained mechanism which causes its contacts to operate at a certain specified rate. Thus, the oscillators above referred to have contacts which operate at the rate of 75, 120, and 180 pulses per minute, respectively.

In the traflic diagrams of Figs. 2A to 2J, inclusive, an arrow with a solid line shaft is used to designate the direction and presence of a driven code; whereas, an arrow with a dotted line shaft is used to indicate the presence and direction of an inverse code. The particular number appearing near these arrows in the case of the driven codes, indicates the code rate which is represented by that arrow. In the case of the inverse codes, the polarity of such inverse code pulses has been indicated by the sign or adjacent to the arrow indicating an inverse code. An elongated loop indicates that no code pulses are being applied to the associated track section in that direction.

Reference to Figs. 1A to 1G, indicates that there is at each signal location some apparatus which performs a function similar to that performed by corresponding apparatus at each of the other signal locations. Therefore, these similar devices have been given the same letter reference characters, but the devices are made distinctive by reason of the numerals which precede the letter reference characters, the numerals in each case referring to the particular signal with which that apparatus is associated.

Each of the coded track circuits in this circuit arrangement is assumed to be of the conventional direct current type using either primary batteries or trickle charge storage batteries as a source of power for the track current. It should be understood, however, that the track circuits might as well be energized from alternating current sources.

Fig. 1A shows that there is connected across I the rails of track section IT, a track relay 2TB,

through a back contact I84 of the inverse code transmitting relay ZCT. Track battery T32 is illustrated as being connected to the track sec tion 2T through front contact I84 of this same relay 2CT and through either front or back contacts I82 and I83 of relay 2P0. The polarity of the code pulses thus applied by the battery T32 to the track section 2T is dependent upon the position of contacts 182 and I83 of pole changing relay 2P0. If, for example, relay 2P0 is picked up so that front contacts I 82 and I83 are closed,

positive pulses, i-. e. top rail positive with respect to the bottom rail, will be transmitted; whereas, if the relay 2P6 is dropped away, track battery T132 will be applied to the rails. through back contacts I82 and I 83 and the applied code pulses will be of negative polarity. A resistor It is shown as being connected across the track rails for the purpose of suppressing any large transient, inductive current which mig ht beproducedduring conditions of high ballast, resistance by a train leaving the track section 2T." Another resistor is included in series with the track relay 2TB. This resistor Iiispreferably of the variable type so that an adjustment may be made for varying trackcircuit constants. Similarly, a resistor I6 is included in series with the track battery TBZ to permit an adjustmentof the current applied to the rails of; the track section. A condenser I1 and resistor I8 are connected in series across the front contact I I of the inverse code transmitting relay 2CT for the purpose of reducing the arcing of this contact as the track battery is intermittently connected to the rails of the track section.

In addition to an inverse code transmitter relay CT being provided for each track section at a signal location, there is also provided a relay CCA, such, for example, as the relay 200A of Fig. 1A, for governing the duration of the respective inverse code pulses.

The various signals employed'in this signalling system, such as the head-block signal 2 in Fig. 1A may be assumed to be of the searchlight type such as shown in the O. S. Field Patent No.

repeater relay such as relay IYGP. All of the signals shown in this embodimentare approach lighted and, therefore, do not actually display an aspect unless there is a train in approach of them. Assuming, however, that a signal is illuminated, the signal apparatus has been diagram- I matically arranged so that if the polarity of the voltage applied to theoperating winding of the signal is positive, 1. e. the top of the block. marked SA with respect to its bottom, the signal displays a green aspect; whereas, if the polarity applied to such winding is negative, the signal will display a yellow aspect. If no voltage at all is applied to the operating winding, the signal displays a red aspect.

vThe movable contacts and 2| associated with the signal 2 operate in such a manner that they are in left-hand and right-hand positions, respectively, when the signal. 2. is controlled to display a red aspect; both. inright-hand positions when the signal is controlled, to. display a green. aspect; and both in left-hand positions, when the signal 2 is controlled to display a yellow aspect.

Although the above description has been directed particularly to the signal 2 shown in Fig. 1A, these remarks apply to all of the other signals shown as well since all ofthe signals are assumed to be of the same type. Furthermore,

although. searchlight:v type signals: are shown in this embodimenthanyg other type oifsignel such as a color-light. or. QOSltiOheHEh-t. signal, might as well be used:

OPERATION General When Figs. 1A and. 1E" are arrangedend-toend it will. be noted; that. there. is a cut-section provided between each of the various signal locations. As has. briefly been described, each of these cut-sections includes apparatus whose function it is to. apply driven code. pulsesto. the. track sections adjoining such. cut-section. Thus, the cut-section. apparatusjshown 13 operates to apply drivencod'e. pulses to track sections 21. and. 5T.. The code transmittingrelaysZC'I and 501, of course, operate. independently inthat the codesappliedto the two adjoining track sections may be. either of the same or difierent rates. Since each of these cut-sections are, identical with regard to their apparatus and circuit, only one of' these cut-sectionshas. been shown in detail (see Fig. 1B). The other. cut-sections have all been shown in block formats, for instance the cut-section I10. located between, the ends of passing siding B;

At each of the signal locationsthere are located inverse, code transmitters whichtransmitinverse transmittersare of'the. conventional type and are representative of the inverse code, transmitters tobe found at. all the signal locations. The rectifiers I11 and, I18 areincluded in thecircuits to relays: 2CCAand. 2CT to prevent an inductive voltage surge from picking up these relays. when the back contact I16 of track relay ZTR opens. 'I'hecircuit organization for thecontrol.of'energization of the inverse code transmitter relay CT is thereforesuch that each relay CT is picked up upon the dropping away of the associated track relay, TR, and the termination. of; its energization is inaccordance with the adjjustmentof the variableinductance in seriesv with the winding of the associated relayCCA. More specifically, for example, with reference to Fig. 1A, the relay ZCT becomes pickedup upon the closure ofback contact I16 of relay 2TB by the energization of acircuit including back contact i relay ZCCA. Energy is also applied to relay ZCCA at the same time through back contact" I16 of'relay 2TB, but the picking up of-relay-ZCCA is delayed'in accordance with, the adjustment of the variable inductance 19. When relay 200A becomes picked up, the inverse pulse is terminated by the opening of back contact 1Il in' the circuit for relay 2'C'I';

Cut-section (Fig. 1 B) rowszinclosedwithimtheconventional relay s-y mbol. For example, it the polarity of the voltage applied to the track section 2T is positive with respect to the bottom rail, the relay ZDTR will be picked up.

The circuits at each of the cut-sections are operated in accordance with the polarity of the inverse code pulses received there. Thus, when positive inverse code pulses are received at the cut-section shown in Fig. 13, each picking up of front contact 25 of relay ZDTR, energizes the track repeater relay 2DP through a circuit which includes in addition to contact 25 of the relay 2DP, a back contact 24 of track relay 2HTR. Relay 2DP is provided with sufiiciently slow release characteristics to enable it to remain picked up even though it is energized only intermittently when positive code pulses are received. During each off period of this inverse code the relay 2CC is energized through a circuit including back contacts 24 and 25 of relays ZHTR, and ZDTR, respectively, and front contact 38 of the track repeater relay 2DP. It will be noted that this relay 2CC is also provided with slow release characteristics so that it remains picked up even if it is also energized only intermittently during each off period of the inverse code. The relay ZCC acts as a back contact repeater of both track relays 2DTR and 2HTR and insures that the desired operation of the circuit organization will take place only when the energy received at this signal location consists properly of code pulses and not of stray or transient voltages.

When the inverse code pulses received over the track section 2T are of negative polarity, the relay ZHP will be picked up by the intermittent closing of front contact 24 of track relay 2HTR. During each off period of this inverse code, the relay 2CC will be energized through a circuit including back contacts 24 and 25 of relays 2HTR and 2DTR, respectively, and front contact 26 of relay 2HP. Thus, although the relay 200 is picked up by both positive and negative inverse codes, the track repeater relays 2H? and 2DP are selectively energized in accordance with the polarity of the code pulses being received.

The selective picking up of the relays 2H]? and 2DP results in the energization of the proper code oscillator so that code pulses of the proper rate may be applied to the adjoining track section T. Specifically, the reception of positive inverse code pulses at the cut-section shown in Fig. 1B energizes the 180 code oscillator 5-|8||CT through a circuit which includes front contact 21 of relay 2DP, and when negative inverse pulses are received, the 120 code oscillator is energized through a circuit including front contact 28 of relay 2HP. When no code pulses are received, the rate code oscillator is energized through back contacts 21 and 29 of relays 2DP and 2HP, respectively. In this manner, any one of the three code oscillators at this location may be made operative as required.

The code transmitting relays at each cut-section are energized in a manner which is also dependent upon the polarity of the received code pulses. Thus, if the inverse code pulses received from the signal 2 location are of positive polarity the coding relay SCT is energized at the rate of 130 times per minute because, in that case, relays 2DP and ZCC will be picked up, and relay SCTA will be energized through a circuit including front contact SI of relay 206, front contact 32 oi relay 2DP, the front contact 33 of the 180 code oscillator S-IBUCT and back contacts 34 and 35 of relays BDTR and BH'I'R, respectively. Since the contact 33 of the 180 code oscillator opens and closes at the rate of 180 times per minute, relay 5CTA will pick up and drop away at this rate so as to apply a 180 driven code to the rails of track section 3T.

By including back contacts 34 and 35 of relays SDTR and SHTR, respectively, in the circuit for code transmitter 5CTA, a back contact check of the inverse code is provided for as disclosed in the Preston patent, No. 2,371,263, dated November 9, 1940.

If the inverse code pulses received at the cutsection shown in Fig. 1B are of negative polarity, the coding relay 5CTA will be energized through a circuit including front contact 3| of relay 2CC, back contact 32 of relay 2DP, front contact 38 of relay ZHP, and front contact 31 of the code oscillator 5-I20CT so that the relay SCTA will be energized 120 times per minute. When no code pulses are received at this cut-section, the coding relay ECTA will ordinarily be energized through a circuit including back contact 3| of relay 2C0 and a front contact 39 of code oscillator 5-15CT so that the driven code applied to the rails of track section 5T will be of the rate.

The circuit organization at this cut-section location is so arranged as to permit a rapid transfer of the driven code rate under certain conditions. Thus, if we assume that a positive inverse code has been received over the track section 2T but that these inverse code pulses are suddenly no longer received due to the approach of a train, both of the track relays ZDTR and ZHTR will remain deenergized and have their back contacts closed. Assuming that the previously received code pulses were of positive polarity, a circuit will now be closed to energize relay 26C through back contacts 24 and 25 of the track relays ZHTR, and ZDTR, respectively, and through front contact 38 of relay 2DP since this relay 2DP will remain picked up for an interval corresponding to its slow release time. When the repeater relay 2DP does finally drop away and open its front contact 38, the circuit to relay ZCC will be opened so that this relay will also become deenergized. Obviously however, these relays 2DP and 20C must each have a release time greater than the ofl periods of the slowest inverse code, and since, as has just been demonstrated, relay 200 does not drop away until after relay 2DP has dropped away, the above described circuit by means of which relay 5CTA is energized at the 75 rate cannot be established until after a period of time corresponding to two off periods of the driven code. Thus, in order to insure that the driven code transmitter SCTA can be energized during this interim period, an alternate circuit is provided to energize this relay 5CTA through front contact 3| of relay 200, back contact 32 of relay 2DP, and back contact 36 of relay ZHP. Then when relay 200 does finally drop away, the previously described circuit for relay ECTA which includes back contact 3| of relay 200 is established. In this manner, a rapid transition is made and the interruption of the driven code applied to track section ET is avoided.

From the foregoing description it can be seen that the rate of the driven code applied to the track section to one side of the cut section is dependent upon the polarity of the inverse code received over the track section to the other side of the cut-section and that, when no inverse code is received, a 75 rate carrier code is then applied with. the result that inverse codes. may be. used in this signalling system. with: a.,max-im,u ma of: safety.

Normal. conditions- (Eim 2A)? signalling system of the present invention, it is believed desirable to point out the manner inwhich the apparatus is controlled and thevarious controls transmitted under conditions of'no traffic.

As shown in Fig. 2A all ofthe signals included inthis signalling system are normally conditioned to display a green aspect. Thus, beginning with the head-block signal location shown in Fig. IF, it is evident that since signal IOis conditioned to give a green aspect, yellow-greenrepeater relay WYGP will beenergized through contact I05 associated with the signal mechanism. As a result, front contacts 14 and 15 of this. relay IUYGP will be closed and'positi-ve inverse code pulses will then be applied to the rails of track section. HT. These inverse code: pulses is in its normal position so that contact 80 as-' sociatedv with the switch ZW' circuit" controller will be closed thereby permitting an alternatingvoltage to be applied to the primary of transfcrmer H16 due to the intermittent picking: up and dropping away of contact MO of track relay 8TB. Two decoding units. are provided and these are indicated in the drawing by the blocks, labeled IZODU and I80DU. If the driven code received over track section 8T is of the 120 rate, relay 8H will be picked up; whereas, if the driven code is of the 180 rate, relay 8D. will be picked up. In addition, however, an auxiliary circuit is provided for relay 8H which includes a front contact I09 of relay 813. so that, whenever relay 8D is picked up because of there being a 180 code received at this location, relay 8H will also be picked up. The. decoding units t20DU and l80DU are of the conventional type and include a full-wave rectifier. Thus, the winding, of relay SH is connected across the output. terminals of this rectifier. For this reason the auxiliarycircuit for relay 8H described above. is so are ranged with respect to its: polarity and this rectifier that a short-circuit; cannotoccur. In addition. resistor I19 is included to. prevent. execs! sively high currents should breakdown .of the rectifier occur. Under the normal conditions now being considered, when a 1.80 code is received as has just been described, both vrelays, 8D and 8H are picked up. As a result of this con.- dition, front contact III of relay SHisclosedso that a circuit is completed to-energizc signal 8. Back contact 69 of relay 95B is, of course. now closed since these directional stick relays are picked up only by the passage of trains pasta signal location as will presently be described.

Also, since relay. piokediu a front.

acts I I12 and: I13; closed and: theivo t se a p i d. otha operat ngwi din fsi nal 8 in,

' herefore. Qfipos tive polarity and will. conch-.-

i o; hisa ianal. Brio,- display; a. green aspect.

With. si nal-.- 5" conditioned. to. displays a green aspect; andjwith, relay: aclhpickedrup, .du tothere bein driven codepnls arec iv d ver track s e- As an aid in understanding the operation of the glue Waterway-1GP Wm gized through a, circuit including front. contact; I-l;4 cf'relar; 90R, and-contact. Isl-:5 associated with the i nal: 8;.lncchanismi. Thus, relag;.8G?-.-will*be pi ked up: and-,. duesto he: closing: or ita-firont con-s tact H6; will ener ize: pole changing relay WC:

fionsequentlvi fr nt contacts 11 and 1a or this. relay" 8PC1W111 be closed end-a the. inv rse.v code; ransmitted; over track; sectional will beof osh. t ve p laritytand willzcanse a-.1-8,0:driven;code;to1

a be; received; at; the; signal; 6:. location... shown. in

The, circuit. associated. with this intermediate 7 in. that the 1.80: code receivedover track section,

6T will: cause relays: BER; 8H,: and 6D to. become, energized. Consequently, front contact. 'lillaof re: laywill be closed, and, sinceback; contact; l.l;9 of relay 1 1SR- will: be. closed under these. cilcuma .1 stancea, a. circuit. will be completed; to energize.

i nal. 6. Due.to 'the fact; that. relay. &D.is pi.ckedr upandits front. contacts. lit. and. i211 closed, thisv voltage applied to, signal; It wilt be. positive and will condition this si nal to give a. green. spect.

: A circuit will: then. be completed. through. contact 1:23 associated with the. signals mechanism. to. energize yellowsteem repeater relay- 685GB, With this relay' fiYGP picked up, its; front con tacts I24; and lzs-wlll be closed-and will result the. transmission. or a, positive inverse code over the railsv of track; section 1 T.

In a, manner similar to that: which ha just.

been described, each 01 the various. intermediate,

signals governingeast-hound tralfic willybe ditioned to display a men aspect. and will; caus the inverse, code transmitted westwar each or these intermediate signal; locations. to be. of positive; polarity. The result. w nibflthat-a. 1 .1 driven code. will be received at. the headloclc signal location shown in Fig. 1A over the. section IT and will, smaller-1v, causerelays ICE. 2-H, and ID to be picked up. A similar circuit. will then again be completed to supply the op rating; mechanism of signal 2 with a voltage of positive polarity so that this signal. will also bezcondla.

timed to. display a green aspect. By these means all or the signals overnin cash. bound trafllc are conditionally controlled to displaygreen aspects and, since the circuits are sym: metrical, it is obvious that all of the. Signals governing westbound traflic will besimi'larly controlled, Thus an organization is provided em. playing both driven and inversecode with. the source of the driven code. being located in each case at a cutssection intermediate between the signal locations so organized that, under normal conditions, the drivencodes will all :be of-the rate and the inverse codes will all be. of positive polarity.

lzirectiozml stick Islet examlnet-iono .14 to 1G inc usive, po nts out the f ct that. the control c cui :tcr each wayside signal includes acontact of a dime.-

tional stick nela lh re ore, i order tocbteln a better understanding of the way in which these 11 signals are controlled, the operation of these stick relays will now be explained.

Each of the directional stick relays at a signal location is picked up by the passage of trains in a particular direction. More specifically, a train passing the head-block signal location shown in Fig. 1A in an easterly direction will cause the picking up of stick relay ZSR while the passage of a train past this same location in a westerly direction will result in the picking up of stick relay 3SR.

Fig. 1A shows that there is associated with the directional stick relay 38R, a yellow-green repeater relay 3YGP and a green repeater relay 3GP. Both of these signal repeater relays are slow to drop away and this is indicated in the drawings by the heavy line bases of the symbols for these relays. If signal 3 is conditioned to display a green aspect, green repeater relay 3GP will b energized through a circuit including front contact 4| of relay ZCR. and contact 42 associated with the signal 3 mechanism. Yellow-green repeater relay 3YGP will then be energized through a circuit including front contact 43 of the relay 3GP and contact |85 of the switch |W circuit controller. If signal 3 displays a yellow aspect, relay 3GP will be dropped away but relay 3YGP will be energized by a circuit through front contact 4| of relay 20R and contacts 42 and 44 of the signal 3 mechanism. In either event, relay 3YGP will be picked up and its front contact 45 closed. Then as soon as a westbound train enters track section 2T causing relay 20R to drop away, a circuit is established to energize relay 3SR from back contact 4| of relay 2CR, front contact 45 of relay 3YGP, the winding of relay 35R, to The entrance of the train into track section 2T and the resultant dropping away of relay 2CR deenergizes relay 3GP if it has been picked up because of the opening of front contact 4| of relay 20R. When relay 3GP drops after an interval corresponding to its slow release time, the opening of its front contact 43 deenergizes relay 3YGP while the closing of its back contact 43 establishes a stick circuit for directional stick relay 38R including contact I85 of the switch circuit controller and front contact 46 of relay 3SR. Relay 3YGP, because of its slow release characteristics, does not immediately drop away upon being deenergized by the opening of front contact 43 of relay 3GP so that there is sufficient time for the above described stick circuit of relay 3SR, to be established before the pick-up circuit of this relay 35R is interrupted by the opening of front contact 45 of relay 3YGP. If the relay 3YGP has been energized through contacts 42 and 44 of the signal 3 mechanism as previously described, this relay will now still be dropped away by the opening of front contact 4| of relay 2CR.

As a result of the stick circuit for relay 3SR being established, relay 3SR. will remain picked up until relay 3GP is again picked up, opening its back contact 43 or, until contact I of the switch circuit controller is opened by the operation of switch IW from its normal position.

Another directional stick relay 2SR is associated with signal 2 and is picked up by the passage of eastbound trains as has briefly been explained. Thus, a train approaching the signal 2 location will encounter either a green or yellow aspect from the signal 2 if the track ahead is clear for the required distance. Therefore, the yellow-green repeater relay 2YGP will be energized through a circuit including contact 20 asthis reason, back contacts 41 and 48, respectively,

of these relays ZCR and 30R will be closed so that the directional stick relay ZSR will be picked up through a circuit including these contacts and front contact 50 of relay ZYGP. The presence of the train in track section 2T will cause the deenergization of signal 2 because of the opening of front contact 5| of relay 2H. Thus, with contacts 20 and 2| associated with the signal 2 mechanism in their left-hand and right-hand positions, respectively, the yellowgreen repeater relay 2YGP will be deenergized and the closing of its back contact 50 will establish a stick circuit for the relay 2SR through its front contact 52. Because of the slow release characteristics of relay 2YGP the entrance of the train into track section 2T will not result in the dropping away of this relay until the above described pick-up circuit of relay 2SR has been completed. This relay 2SR will remain picked up until the signal 2 again is conditioned to display either a green or yellow aspect which will permit the picking up of relay 2YGP and the opening of its back contact 60 to open the stick circuit.

Train movements (Figs. 2B-2J) In order that a better understanding may be obtained as to the mode of operation of this signalling system, the operation of the circuits will be described for various conditions of traffic. The conditions of the various signals and the rates and polarities of the various codes are shown for a number of different traffic conditions in Figs. 2B to 2J, inclusive. Although specific reference will be made in the ensuing description to the circuit organization shown in Figs. 1A to 1G, inclusive, these traffic diagrams comprise a helpful adjunct to this description as they clearly show the condition of this signalling system corresponding to each of the traffic conditions to be discussed herein.

To maintain a logical order with respect to this description, a train designated TNI in Figs. 2B to 2J is shown as advancing eastwardly along the portion of single track railroad shown, which corresponds of course, to the portion of track shown in Figs. 1A to 1G.

In Fig. 2B the eastbound train TN| is shown as having entered the track section 3T opposite the east end of passing siding A. A comparison of this Fig. 213 with Fig. 2A shows that the opposing signal 3 is now conditioned to display a red aspect and that the opposing signal 5 is now conditioned to display a yellow aspect. In addition, signal 2 is now illuminated and displays a green aspect.

The reason that the opposing signals 3 and 5 are now conditioned to display the aspects shown in Fig. 2B is that the shunting effect of the train as it occupies track section 3T, causes the dropping away of relays 3D and 3H. Thus, with front contact I43 of relay 3H open, the circuit to the operating winding of signal 3 is interrupted and this signal is, therefore, now condi- .13 tioned to display a red aspect. As a result, neither green repeater relay 3GP nor yellowgreen repeater relay 3YGP can now be energized through contacts 42 and 44 associated with the signal 3 mechanism. For this reason, both front contacts I44 and I45 of these relays 3GP and 3YGP respectively, will be open so that pole changing relay 2P0 can now not be energized. Back contacts I82 and I83 of this relay'ZPC will, consequently, be closed so that the inverse code pulses applied to track section '21 will be of negative polarity. The driven code received at the intermediate signal location will, as a result, be of the 120 rate.

At this signal 5 location, shown in 1G, the relay 5H, will be picked up due to the "120 code being received but the relay will be dropped away. Thus, although front contact IZ lof relay SE is closed so that a circuit .is completed to energize the signal 5 operating winding, back contacts I46 and I4! of relay 513 will now be closed so that the voltage applied to the signal 5 operating winding will be .of negative polarity and will condition this signal to display a yellow aspect.

The illuminated condition of signal 2 caused by the presence of the train'TN! in tracksection 3T is produced as .a result of the dropping away of code receiving relay 3GB which causes back contact 51 of this relay to be closed seas to apply energy to the .lamp L2 of signal 2. As a result, this signal 2 .now displays .a green aspect, -thereby signifying that the train TN I is .free to enter the stretch of single track between passing sidings A and B.

As was previously mentioned, a tumble-down occurs when a train enters the stretch of single track between head-block locations. This condition is shown in .Fig. .20 in which the train is shown as occupying the tracksection 2T. Before examining in detail .the manner which this tumble-down is brought about, it will be noted that, since the train now shunts the rails of track section 2T, relays 20R, 2H and 213 will be dropped away. Consequently, front contact 5 of relay 2H will now be open thereby deener gizing signal 2 and, since back contact All of relay ZCR, is now closed so as to energize the lamp L3 of signal 3, this signal now displays .a red aspect.

The passing of the train .TNI from track section 3T into track section 2T has caused directional stick relay ZSR. to become energized in a manner which has already been described. Thus, even though .frontcontact 3B of relay 20R is now open, an alternate circuit is now ,provided through front contact 49 :of' relay .ZSR to energize the inverse code transmitter 3C'I. And, since signal .2 is now conditioned to display a red aspect, yellow-green repeater relay ZYGP is now dropped away so that its back contacts I80 and I-8I are closed thereby providing that the inverse code pulses transmitted by inverse code transmitter -3CT are of negative polarity.

Under the conditions shown in Fig. 0, no inverse code pulses can be received at the cut section location shown in Fig. 113 because of the occupancy by the train of .track section .ZT.

Thus, according to the description of the mode ofoperation of the circuit organization at a cut section as already set forth, a driven code of the 7.5 rate will now be transmitted toward .the intermediate signal location .shown in Fig. 1C.

At this signal 5 location, the code receiving relay 5GB will be picked up by th action of codeflaut relays 15D and "5Hrwill'be dropped away. ,a'result. front contact I24 of relay 5H will be opened so as to open the circuit to signal 5 .and condition this signal to display a red'aspect. Yellow-green repeater relay SYGP will, therefore, drop away :and open its front contact 68. therebydeenergizing the inverse code transmitter ICT. 'No inverse code pulses will then be transmitted over the rails of track section IT anda'75 driven code will, therefore, be received at the intermediate signal location shown in, Fig. 1B .and will condition signal 1 to display a'red aspect. Similarly, all of the intermediate signals governing westbound trafiic will be conditioned todisplay .a red aspect and each oi the inverse. code transmitters at these intermediate signal locations which ordinarily transmits an inverse code eastwardly from such intermediate location, will be .made .inactive. Consequently, a '75 drivenwcode will also be received at the signal 9 head-block location shown in .Fig. 1E. 7

.At the head-block location shown in Fig. 1131, the reception of a7f5 rate driven code will cause code receiving relay R to remain picked up but willcause both relays 9D and 9H to drop away. For this reason, front contact I25 of relay 9H will be opened thereby deenergizing signal 9 and conditioning this signal to display a red aspect. "Howeven-since code receiving relay 90R isstill picked up, inverse code transmitter BCI' will still be energized through .front contact 6| of this relay 90R and, as a result, inverse code pulses will .be applied to the rails of track section BT. Since yellowegreenrepeater relay QYGP will now be dropped away due to the fact that signal 19 isnowconditioned to display a red aspect, back :con'tacts I26 and I2! of this relay SYGP will be closed and the inverse code pulses applied to track section 8T will be of negative polarity. For this reason, the driven code received .at the 'headblock location at the east end of passing siding B (see Fig. 1F) will be of the rate and will cause relay H as well as relay LICR to be ,picked up. Relay II.D, however, will .be dropped away and its back contacts I30 and L3] closed so that, although front contact I32 of relay HHis closed thereby completing a circuit to the operating windings of signal II, the voltage .thus applied to this signal will be of negative polarity and will condition this signal to display a yellow aspect.

At .the head-block location shown in Fig. 1F the inverse code transmitter IUCT'Will still be energized through a circuit including front contact .232 of code receiving relay IIlCR. However, since signal II is now conditioned to display a yellow aspect, green repeater relay HGP can now no longer beenergized through contact I34- associated with the signal II mechanism. This relay I IGP will, therefore, drop away and, in opening its front .contact I35, will open the normal circuit for the energization of yellow-green repeater relay .II'IYGP. This relay IIYGP will now be energizedthrough a circuit including contacts I34 and I36 associated with the signal H mechanism. The opening of front contact I 3'! of relay IIGP also causes the deenergization of pole changing relay MP0 for, although relay IIYGP is now picked up and its front contact I 3.8 closed,rlay I ISR still remains dropped away so that its front contact I39 is open. Thus,'back contacts I 4| .and I42 of ,pole changing relay IIPC are now closed and th inverse code pulses now applied to the rails'of'track section I-IJT are of negative polarity with the result that a driven code of the 120 rate is received at the intermediate signal I3 location shown in Fig. 1G. This 120 driven code will, similarly, cause signal I3 to be conditioned to display a yellow aspect. As a result, yellow-green repeater relay ISYGP will be picked up and the inverse code applied to track section IZT will thus be of positive polarity.

In this manner, a tumble-down of all the opposing signals between passing sidings A and B is produced by the entrance of an eastbound train into the stretch of single track between these passing sidings. In addition, a double caution results for the two opposing signals II and #3 to the east of passing siding B. Since the circuit organization of the present invention is symmetrical, the entrance of a westbound train into the stretch of single track between passing sidings A and B will similarly condition all of the opposing signals to display a red aspect and will condition the two opposing signals just to the west of siding A to display a yellow aspect.

When the eastbound train TNI occupies track section T as shown in Fig. 2D, it will be noted from this figure that the only change with respect to the control of the various signals is that signal 4 is now illuminated whereas signal 3 is now no longer illuminated. This change is brought about by the fact that the train in its movement from track section 2T into track section 5T now causes the relay 5CR to be dropped away instead of the relay 20R. Thus, only the approach lighting of the adjoining signals 3 and 4 is affected because the movement of this train TNI from track section 2T into track section 5T does not effect the operation of either the D or H relay at either the signal 5 location or the signal 2 location. Thus, since the operation of the signals is afiected only by the movement of trains from one signal block into the other except as otherwise mentioned, movement of trains between signal locations, i. e. from One side to the other of a cut-section, will not ordinarily be described in detail.

In Fig. 2E, the eastbound train TNI is shown as having passed the intermediate signal 4 location. With the train thus occupying track section 4T, relay 40R, 4D, and 4H will be dropped away. As a result, front contact I50 of relay 4H (see Fig, 1C) will be open thereby deenergizing the operating winding of signal 4 and conditioning this signal to display a red aspect. Yellow-green repeater relay 4YGP can no longer be energized through either contact I 5| or contact I53 associated w th the signal 4 mechanism so that this relay 4YGP will drop away. The opening of front contact 62 of this relay 4YGP will interrupt the normal circuit for the energization of inverse code transmitters 5CT but, since the passage of the train in an eastbound direction past the signal 4 location has caused directional stick relay ISR to be picked up, front contact '3 of this relay 4SR will now be closed thereby providing an alternate circuit for the energization of inverse code transmitter 5CT. Since back contacts 64 and 65 of yellow-green repeater relay 4YGP will now be closed the inverse code applied to the track section 5T will be of negative polarity and will cause a 120 code to be received at the signal 2 location shown in Fig. 1A.

At the signal 2 head-block location a 120 driven code received over the track section 2T will cause relays 20R. and 2H to be picked up but will cause relay 2D to remain dropped away. Thus, al-

through front contact 5| of relay 2H will be closed so as to complete a circuit for the energization of the signal 2 operating winding, the voltage thus applied to this operating winding will be of negative polarity since back contacts 53 and 54 of relay 2D will now be closed. As a result, this signal 2 will now be conditioned to display a yellow aspect,

Since signal 2 is now conditionally controlled to give a yellow aspect, yellow-green repeater relay ZYGP will now be energized through a circuit including contacts 20 and 2| associated with the signal 2 mechanism. For this reason, front contacts I and I8I of relay ZYGP will now be closed and the inverse code pulses applied to track section 3T will now be of positive polarity.

Because the directional stick relay 4SR is picked up under the conditions shown in Fig. 2E, back contact 66 of this relay will be opened thereby deenergizing the operating winding of signal 5 and conditioning this signal to display a red aspect. In this connection, it will be noted that, at each signal location, the picking up of a directional stick relay by the passage of a train in a particular direction past that signal location conditionally controls the opposing signal to display a red aspect. In this manner, the condition of the directional stick relays is checked to insure that they do not remain picked up at such times as they are supposed to be in their dropped away condition. In the previous description of the way in which the tumble-down operation was produced, it was seen that a necessary step in this procedure was the deactivation of one of the inverse code transmitters by the dropping away of a signal repeater relay. Thus, when signal 5 was controlled to display a red aspect because of there being a '75 rate driven code received over track section 51, the resultant dropping away of yellow-green repeater relay 5YGP was intended to make the inverse code transmitter 4CTA inactive because of the opening of front contact 60 of this relay 5YGP. If, however, relay 5SR had remained picked up after the previous passage of a westbound train past the signal 5 location, the closed front contact I59 of this relay 5SR would have allowed the continued operation of the inverse code transmitter 4CT thereby preventing a complete tumble-down through the various track sections to the east of this intermediate signal 4 location. However, by including a back contact I69 of relay 5SR in the circuit to the operating winding of signal 4, this signal 4 would remain deenergized and be conditioned to display a red aspect until such time as the directional stick relay 5SR would again be dropped away. In this way, the erroneous operation of the directional stick relay is prevented from causing an unsafe condition.

With the train TNI occupying track section 4T, relay 4CR will, as has been stated, he dropped away. Thus, back contact I54 of this relay 40R will be closed thereby energizing lamp L5 of signal 5 so that this signal will now be illuminated.

By analogy from the preceding description, it is obvious that when the train is in track section 6T, there will be a driven code of the '75 rate applied to the track section 9T. This driven code will permit the relay R to be picked up but will, of course, not permit the energization of relay 9H. Thus, signal 9 will be conditioned to display a red aspect and signals I I and I 3 will, as shown in Figs. 20 to 2E, both be conditioned to display a yellow aspect.

In Fig. 2F, the train TNI is shown as occupying 17 the track section 9T- Thisdrawing shows that in this particular case, themovement of a train from one track section to another while still remaining in the same signal block, does affect the signal operation. Thus, in the condition illustrated in Fig. 2F with the train occupying track section 9T, relays 90R, 9H, and 9D will be dropped away thereby causing signal 8 to be com ditioned for the display of a green aspect. A 75 rate driven code will now be applied to the track section GT and will condition signal 6 to display a red aspect. Also, relay EYGP will then be dropped away, opening its front contact I55 out, since directional stick relay BSR will now be picked up, front contact I56 of this relay BSR will now be closed to provide an alternate circuit for the energization of inverse code transmitter ICT. In a manner similar to that already described in connection with the intermediatesignal 4, the inverse code transmitted over track section IT will be of negative polarity so that a 120 driven code will now be received at the signal 4 location. At this signal 4 location, the relay 4H will be picked up but the relay 4D will remain dropped away. Thus, although front contact I54! of relay 41-1 isnow closed to complete a circuit for the energization of the operating winding of signal 4, back contacts I56 and I5! of relay 4D will now be closed and the polarity of the voltage applied to the. operating winding of signal 4 will be negative and will condition this signal 4 to display a yellow aspect.

Since signal t is conditioned to display a yellow aspect under the conditions shown in Fig. 2F, yellow-green repeater relay IiYC-P will now be energized through a circuit including contacts IiiI and its associated with the signal 4 mechanism. For this reason, the stick circuit for directional stick relay 43R will be opened by the opening of back contact I58 of relay 4YG-P so that this relay iSR will now drop away. Thus, back contact 65 of this relay 43R will now be closed thereby allowing a circuit to be completed to the operating winding of signal 5. Since the driven code received at the signal 5 location over the track section 5T is of the 180 rate as shown in Fig. 2F, both relays 5D and 5H will be picked up and the operating winding of signal 5 will, therefore, now be conditioned to display a green aspect.

As has been pointed out, relay SCR is dropped away under the conditions shown in Fig. 2F. With contact SI of this relay open, the code transmitter 8CT cannot be energized and no code pulses can then be applied to track section. 8T. A 75 driven code will. then be received at the signal I I location and this signal will be conditioned to display a red aspect. The pole changing relay PC will still be dropped away so that negative inverse pulses will still be transmitted over track section IUT and signal I3 will, therefore, still be conditioned to display a yellow aspect.

The eastbound train TNI is shown in Fig. 2G. as having passed out of the stretch of single track between passing sidings A and B so that it now occupies track section 8T opposite thewest end of passing siding B. With the train in this position, its shunting effect upon the rails of track section 8T causes relays 80R, 8D, and 81-1 to be dropped away. With front contact III of relay 8H open, the circuit to the operating winding of signal 8 is broken so that this signal now conditioned to display a red aspect. As a result, neither yellow-green repeater relay BYGP nor green repeater relay 8GP can now be ener- 18 gized so that with both contacts I6I and H6, respectively, open, pole changing relay BPC is decnergized. Thus, with back contact I1 and I8 of this relay 8PC closed, the inverse code pulses applied to track. section 9TH; will be of negative polarity. Consequently, a 120 driven code will, be received at the intermediate signal 6 location shown in Fig. 1D,. and in a manner similar to that previously described in connection with the other intermediate signals, this signal 6 will now be conditioned to display a yellow aspect. Similarly, a 18.0 driven code will be received at the intermediate signal 4 location so that this signal willv now be conditioned to display a green aspect.

As was the case when the train occupied track section ET the driven code received at the signal II head-block location when the train occupies track section ST is of the '75 rate. Thus, signal II continues to be conditioned to display a red aspect. As before, relay IIPC remains deenergized. so that a negative inverse code continues to be. applied to track section NT and causes signal I3 to be so energized that it still is conditioned to display a yellow aspect.

Since the train in passing from section. 9T into track section 8T has caused. the directional stick relay 88R. to be picked up, back contact 68 of thisrelay BSR will be opened and signal 9 will, consequently, be conditioned to display a red aspect. Y

,In Fig. 2H, the eastbound train TN I is shown as having passed beyond the east end of passing siding B so that itnow occupies track section I0 1. As a result of this condition, relays IOCR, IIH, and IUD are all dropped away. Thus front I contact I64 of relay IIlH is open so as to deenergize signal I0 whereby this signal I0 is now conditioned to display a red aspect. As a result,

yellow-green repeater relay IOYGP can now not be energized throughcontact I05 of the signal I0 repeater mechanism. so that this relay IiiYGP now drops away. Although front contact I2 or" the code receiving relay IGCR is now open, inverse code transmitter IICT can still be energized, through front contact I3 of relay IIlSR which has been picked up by the movement of the train from track section I ITinto track section IIl'I. Thus, the inverse code transmitter I ICT is made operative to apply negative inverse code pulses to track section III. The driven codereceived at the west end of passing siding B is, therefore, ofthe rate and causes the signal 8 to be conditioned to display a yellow aspect.

At the head-block location shown in Fig. 1E, the stick circuitfor the relay SSR is shown as including a back contact I65 of the green-repeater relay 8GP. Although yellow-green repeater relay 8YGP is, under the circumstances shown in Fig. 2H, energizedthrough a circuit including contacts [I5 and I66 oi the signal 8 repeater mechanism, the green repeater relay 8GP remains deenergized. Thus, back contact I65 of this relay 8GP remains closed and the relay BSR, as a result, remains picked up. For this reason both front contacts I6I and I5 of the relays 8YGP and BSR, respectively, are picked up and completea circuit for the energization of relay 8130. Thus, front contacts T1 and I8 of this relay 8P0 are closed so that positive inverse, code pulses are now applied to track section 9'1 with the result that the intermediate signal 6 is conditioned to display a green aspect.

From the foregoing description, it can be seen that. the circuit orgamzation'with respect to the pole changing relay 81C is so organized as to permit not only the double caution provision as previously described, but also the customary block spacing protection for following traflic. Thus, if signal 8 is conditioned to display a yellow aspect so that yellow-green repeater relay BYGP is picked up but green repeater relay 8GP dropped away, the inverse code pulses transmitted to the rear of the signal 8 will be of such a polarity as to condition that signal to display a yellow aspect, thereby providing for the double caution feature. If, however, under these circumstances relay SSR has been picked up by the passage of an eastbound train, the conditioning of signal 3 to display a yellow aspect will not be due to the overlap control caused by a train entering the stretch of single track to the right of passing siding B but will instead be an indication that the signals to the rear of such eastbound train are to progressively clear. Thus, the circuit is so arranged that the conditioning of signal S to display a yellow aspect, coupled with the picking up of the directional stick relay 88R by an eastbound train, will cause the pole changing relay SP to pick up. In this manner, a positive inverse code will be transmitted to the intermediate signal 6 location and this signal 6 will then be conditioned to display a green aspect.

When the train occupies track section |2T as shown in Fig. 21, the condition of signals 10, H, l2, and i3 are the same as that of the corresponding signals 2, 3, 4, and 5 under the conditions shown in Fi 2E. Since signal I0 is now energized to give a yellow aspect, the relay HIYGP is picked up and positive inverse code pulses are applied to track section HT and re suit in the display of a green aspect by the signal 8.

In Fig. 2J a train TNI is shown about to move out of the west end of passing siding B into the stretch of single track between sidings A and B. When the switch 2W is moved from its normal position to permit this train move, the action of the switch 2 circuit controller causes its contacts 80 and 8! to open the circuits in which they are included. Thus, the contact 80 is controlled to open the circuit to the transformer I06 and to the inverse code transmitter SCT. As a result, relays 80R, 8H, and 8D drop away and the inverse code transmitter 8CT is made inoperative. Since front contact ill of relay 81-1 is now open, the operating winding of signal 8 cannot be energized and this signal 8 will, therefore, be conditioned to display a red aspect. Because of this condition of signal 8, both signal repeater relays 8GP and BYGP will be dropped away and the pole changing relay 8PC will, therefore, be deenergized. Consequently, back contacts TI and T8 of this relay BPC will be closed and the inverse code then applied to track section 9T will be of negative polarity. A driven code of the 120 rate will then be received at the intermediate signal 8 location shown in Fig. 1D so that this signal 6 will now be conditioned to display a. yellow aspect.

Since back contact 82 of the code receiving relay 8CR will now be closed, the lamp L9 of signal 9 will be energized so that this signal 9 will now display a green aspect if the driven code received over track section QT is of the 180 rate. Of course, if the driven code received is of the 120 rate signal 3 will display a yellow aspect and if a 75 driven code is received signal 9 will display a red aspect.

If we assume that a 180 rate driven code is received over track section 9T but that signal 9 has been conditioned to display a red aspect due to the picking up of the directional stick relay 88R. by an eastbound train so as to cause the opening of back contact 68 of this relay 88R, the opening of contact 8| of the switch 2 circuit controller will interrupt the stick circuit for this relay SSR, so that it will drop away. Thus, although the signal 9 will ordinarily not clear until the signal 8 is cleared, the throwing of the switch 2W removes this control and allows signal 9 to give an indication as to the trafiic conditions in the stretch of single track between head-block locations.

Operation (Fig. 3)

In the event that it is desired to use a single intermediate signal location for the stretch of single track between head-block signals in place of the double intermediate signals whose operation as already described, the embodiment shown in Fig. 3 may be used.

Fig. 3 illustrates that there are H and D relays at this signal location whose operation is similar to that of the corresponding relays at all of the other signal locations. Relay 14H, for example, is picked up whenever the driven code pulses received over the rails of track section MD are of either the 120 or 180 rate, whereas the relay MD is picked up only when the code received over track section MT is of the 180 rate. iimilarly, signal Hi can be conditioned to display either a yellow or green aspect only when code pulses of the 120 or 180 rate respectively, are over track section [5T so as to cause the picking up of relay IEH and the closing of its front contact I36. The position of contacts 83 and 84 of relay IED determine the polarity of the voltage applied to the operating mechanism of signal [5. Thus, this signal will be positively energized and conditioned to display a green aspect when relay ISD is picked up and front contacts 83 and 84 of this relay closed while the closing of back contacts 83 and 84 of relay I5D will result in negative energization of signal !5 thereby conditioning this signal to display a yellow aspect. Lamp L15 associated with the signal I5 is approach lighted through back contact 85 of relay MCR so that signal I5 is illuminated whenever there is a train in occupancy of track section MT.

The polarity of the inverse code transmitted over track section [ET is dependent upon position of contacts 88 and 87 of relay MD. if driven code pulses of the 180 rate are received over track section MT so as to cause the picking up of relay I 4D, front contacts 8-9 and 3? of this relay will be closed and the inverse code applied to track section EET will be of positive polarity. If, on the other hand, a 120 code is received over track section MT so that relay 5D will be dropped away, the inverse code applied to track section EST will be of negative polarity.

The polarity of the inverse code applied to track section MT is dependent upon the condition of yellow-green repeater relay Iii GP. Thus, if signal 15 displays a green aspect so that relay ISYGP is energized through a circuit including contact 90 associated with the signal l5 mechanism, front contacts Si and 92 of relay ISYGP will be closed and the inverse code applied to track section MT will be of positive polarity. Similarly, this inverse code will be or positive polarity if signal it? displays a yellow aspect 01, in that case, relay I5YGP will be energized through contacts 90 and 93 associated with the signal l5 mechanism. If, however, signal (5 dis- 

